Study On Numerical Simulation Of Sloshing In Liquid Cargo Tanks And Its Coupling Effects With Ship Motions

Recent years, liquid cargo ships such as LNGC become an important facility fortransportation of liquefied cargoes. Navigating at sea, these ships will suffer from randomsea waves and ship motions. With the liquid tanks of large volumes, the liquid sloshing intanks occurs excited by ship motions. In severe sea states, the amplitudes of ship motionsare large and therefore arouse violent sloshing. This violent sloshing can pose greatimpulsive impact to the local tank structures, and cause their damage, and even harm safetynavigation and operation of LNG ships at sea. So it is of practical importance to study theviolent sloshing in tanks and its coupling effects with ship motions.Based on above situations, in this thesis four related issues are focused on: thenumerical simulation of largely distorted free surface motions and computation ofimpulsive pressures during violent sloshing; the numerical simulation of liquid sloshing inbaffled tanks, and evaluation of effectiveness of different baffles; the measurements offlow fields during sloshing in liquid tanks and corresponding numerical simulations; thenumerical simulation and analysis of the coupled motions between tank sloshing and shipmotions.Considering above issues, the work done in this thesis is as follow:1. The governing equations and the corresponding numerical schemes for threedimensional sloshing in tanksConsidering the different kinds of sloshing excitations, the governing equations for3-D sloshing problem are established, and discretized numerically based on the cartesianstaggered grids by finite difference method, also the coupled velocity and pressure in theequations are solved by the over-relaxed Gauss-Seidel iteration scheme.2. The PLIC-VOF method for three dimensional sloshing in tanksIn cartesian staggered grids, the governing equations for volume fractions are solvedbased on the directional splitting scheme and Lagrange convection algorithm. Applying the numerical algorithm for the governing equations, the PLIC-VOF method is developed toreconstruct free surface, and employed for the numerical simulation of the violent sloshingin tanks.3. The CLSVOF method for the violent sloshing in tanksBased on the PLIC-VOF method, the Level-set method is applied to determine thefree surface normal considering its excellent performance in tracking the free surfaceexplicitly. Because of the PLIC-VOF method with good property in mass conservation, theLevel-set method and the PLIC-VOF method are coupled to develop the CLSVOF method.Finally, this method is employed to simulate the violent sloshing in tanks.4. The aperture technique coupled with the PLIC-VOF method in three dimensionsfor the numerical simulation of the liquid sloshing in tanks with the bafflesThe3-D aperture approach is introduced, and therefore the governing equations aremodified to adapt to the change of the volume fraction in cells. Coupled with PLIC-VOFmethod, the sloshing problem in tanks with different type of the baffles is simulated, andthe effectiveness of the baffles are compared and evaluated according to the time historiesof the local pressures and wave elevations.5. The detailed measurements of the flow fields in the tank with the sloshing problemby PIV technique and the numerical simulation correspondinglyThe motion mechanism for the tank is rational designed, and its real-time velocity iscontrolled by the control systems. Considering different filling levels, the excitationfrequencies of the sway motion are set as the resonant frequencies under different fillingconditions. Based on these conditions, the PIV technique is applied to measure the flowfields and the flow patterns in the liquid tanks. Finally, the numerical simulation is carriedout by the PLIC-VOF method and compared with the PIV experimental results.6. The WC-MPS method for the numerical simulation of the tank sloshingWith the weakly compressible assumption, a weakly compressible moving particlesemi-implicit method (WC-MPS) is introduced based on the Tait’s equation of state of thefluid. For this method, the pressure field is obtained directly by Tait’s equation of statewithout solving time-consuming Poisson’s equation. Employing this WC-MPS method, theviolent sloshing with highly-distorted free surface in tanks are simulated and validated withexperimental results.7. The study on the coupling effects of tank sloshing and ship motion in waves Considering the coupling effects of the tank sloshing and the ship motions, thecoupled motion equations of the liquid cargo ships are established. To obtain the force andmoment induced by the tank sloshing, the PLIC-VOF method for vicious fluid is applied.While, to determine the hydrodynamic coefficients and wave loads for the ships in waves,a two dimensional strip theory is applied in frequency domain. Taking advantage of theNewmark-β approach, the coupled motion is solved in time domain to simulate themotions of liquid cargo ships in waves.The creativity of the studies in this thesis can be summarized as the flowing threeaspects:1. The aperture technique coupled with the3-D PLIC-VOF method is developed andemployed to simulate the liquid sloshing in the baffled tanks.The3-D aperture approach is introduced to express the solid phase in the flow field,and the governing equations are modified correspondingly coupled with the PLIC-VOFmethod. And this approach is used to simulate the sloshing problems in the baffled tanks.2. The CLSVOF method in cartesian grids is developed and applied to simulate theviolent sloshing in tanks.For the PLIC-VOF method has good property in mass conservation, and the Level-setmethod possesses excellent performance in tracking interface explicitly. Therefore, theLevel-set method and PLIC-VOF method are coupled to develop the CLSVOF method. Ineach time step, the reinitialization of Level-set function is based on the constructed freesurface by PLIC-VOF method, and the values of the Level-set function are obtained bydetermining the shortest distance from the neighboring cells to the free surface cells. Andthis coupled method is employed to simulate the violent sloshing in tanks.3. The WC-MPS method based on weakly-compressible assumption is developed andapplied to simulate the violent sloshing in tanks.For fully incompressible MPS method, the pressure field is obtained by solvingtime-consuming Poisson’s equation. With the weakly compressible assumption, a weaklycompressible moving particle semi-implicit method (WC-MPS) is introduced based on thepressure field computed directly by Tait’s equation of state of the fluid. Employing thisWC-MPS method, problems of the violent sloshing in tanks are simulated.